The present invention concerns new heteropolymetallic clusters containing palladium and iron, their synthesis, and catalysts made utilizing the clusters.
As used herein, the term "clusters" means molecular structures comprising at least three metallic atoms connected to one another by metal-metal bonds, with these structures possessing ligands which, by their structure, more or less stabilize them. Such kinds of clusters are especially advantageous in preparing catalysts by impregnating them on a support, followed by thermal treatment and reduction in order to obtain metallic particles.
Mixed complexes of the general formula: EQU Pd.sub.2 (Fe).sub.x (CO).sub.4 [CO(NO).sub.2 ].sub.y (dppm).sub.2
with x=2 when y=1 and x=1 when y=0; "dppm" designating the ligand bis(diphenylphosphino)methane, can thus be considered as clusters in conformity with this invention.
The association of iron and palladium permits obtaining interesting results in certain catalytic reactions and it is then important that the distribution and the dispersion of the metals be the best possible. In the case of bi-metallic catalysts containing supported iron, the structure of the catalyst and the state of the valence of the metals are obviously primordial. C. H. BARTHOLOMEW and M. BOUDART have shown in the JOURNAL OF CATALYSIS, 29, 278 (1973) that the stages of impregnation, of heat treatment, and of reduction, as well as the nature of the support, are very important in order to obtain highly dispersed catalysts. In fact, if the metal/support interaction is weak, the metallic particles produced can easily migrate to the surface and form large crystallites thus on silica, if one prepares a catalyst by impregnation of an aqueous ferric nitrate solution, the metal/support interaction is weak and leads to large crystallites, of a mean size of 15 .mu.m, as L. GUCZI and coworkers have shown in the JOURNAL OF CATALYSIS, 60, 121 (1979) and R. L. GARTEN in MOSSBAUER EFF. METHOD, 10, 69 (1977). The fact of starting out from metallic salts in aqueous solutions makes the pH phenomena important which affects the interaction between the surface and the ion and can lead to precipitations at the surface. The metal/support interaction is much more important in the case of alumina as shown by R. L. GARTEN and D. F. OLLIS in the JOURNAL OF CATALYSIS, 35, 232 (1974) and one can prepare a highly dispersed catalyst. On the other hand, by starting from Fe (III), the reduction into Fe (O) cannot be totally effected. One possibility of producing a catalyst in which the iron is highly dispersed and in a state of oxidation near O thus is to deposit iron of low valence on the support. This can be achieved by using carbonyl metals as shown by A. BRENNER in the JOURNAL OF MOLECULAR CATALYSIS, 5, 157 (1979) in J. M. BASSET and R. UGO in ASPECTS OF HOMOGENEOUS CATALYSIS, vol. II, 137, Reidel (1976). In the case in which one searches for bi-metallic catalysts, one sees the advantage one has by starting from heteropolynuclear types such as mixed clusters in which the ligands bestow low oxidation states on the metals. In fact, conventional bi-metallic catalysts are prepared by impregnation with an aqueous salt solution. The impregnations of salts can, moreover, be simultaneous or successive, but there again modifications of the pH during the course of impregnation can provoke processes of metallic aggregation and thus a poor dispersion of phases. In addition, the adsorption velocities generally are not identical and in the event that impregnation takes place on pre-formed supports, concentration gradients appear which are different for the two metals.
These drawbacks disappear with bi-metallic molecular clusters probably because they conduct to well-dispersed and uniform heterometallic phases, with the impregnated clusters capable of keeping the geometry of the starting complexes. The formation of bi-metallic surface aggregates from standard impregnations of derivatives of platinum or palladium and of iron has, however, been noted and described by J. J. BURTON and R. L. GARTEN in ADVANCED MATERIALS IN CATALYSIS, Academic, New York, 33 (1977). But in this case one is not master of the stoichiometry of the types formed. On the contrary, the fact of starting with molecular complexes in which the metal-metal bonds are stable makes it possible to better control the arrangements formed and their stoichiometry.
Very few molecular structures containing palladium-iron bonds have been characterized to the present day. There can be cited the complex of [FePd(.mu.Cl)(.mu.PPh.sub.2)(CO.sub.4 ].sub.2 prepared by B. C. BENSON, R. JACKSON, K. K. JOSHI, and D. T. THOMPSON after their description in CHEMICAL COMMUNICATIONS, 1506 (1968), as well as the types [Fe.sub.4 Pd(CO).sub.16 ].sup.2-, [Fe.sub.6 Pd.sub.6 H(CO).sub.24 ].sup.3-, and [Fe.sub.6 Pd.sub.6 (CO).sub.24 ].sup.4- described by G. LONGONI, M. MANASSERO, and M. SANSONI in the JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, vol. 102, 3242 (1980). In fact, only the three last complexes respond to the definition of clusters given at the start of this description and possess palladium-iron bonds stabilizing the molecular structure, but it should be noted that we are dealing with ionic types. This factor can hurt or complicate their eventual use as catalysts or precursors of catalysts, because one cannot avoid the simultaneous introduction of the corresponding counter-ion, which results in involving a supplementary parameter from which one cannot be freed.